morganlefay,

...and then there is that IRQ that bridges to a higher #??..used to be 9.
Might you be speaking of: IRQ 9 steering into IRQ 2.

In regards to IRQ Sharing, otherwise called IRQ Steering,

IRQ Steering prior to XP
PCI bus IRQ steering started with Windows 95 OEM Service Release 2 (OSR2). It was also supported in Windows 98, Windows 98 Second Edition, and Windows Millennium Edition (Me). By using PCI bus IRQ steering, Windows can dynamically assign or "steer" PCI bus IRQs to PCI devices.
If PCI bus IRQ steering is disabled in Windows the BIOS assigns IRQs to PCI devices, but if PCI bus IRQ steering is enabled, Windows assigns IRQs to PCI devices. When IRQ steering is enabled the BIOS still assigns IRQs to PCI devices and, even though Windows can change these settings, it generally does not.

IRQ Steering with XP
Windows XP cannot rebalance resources in the same way that Microsoft Windows 98 does. After PCI resources are set, they generally cannot be changed. If you change to an incorrect IRQ setting or I/O range for the bus that a device is on, Windows XP cannot compensate by rebalancing the resource that was assigned to that bus.

Windows XP does not have this ability because of the more complex hardware schemas that Windows XP is designed to support. Windows 98 does not have to support IOAPICs, multiple root PCI buses, multiple-processor systems, and other highly complex hardware schemas.

When you are dealing with these types of hardware, rebalancing becomes risky and therefore is not implemented in Windows XP except for very specific scenarios. However, PCI devices must be able to share IRQs. Generally, the ability to share IRQs does not prevent any hardware from working. I believe the Plug and Pray operating system settings in the computer BIOS do not generally affect how Windows XP handles the hardware. However, Microsoft recommends that you set the Plug and Pray operating system setting to No or Disabled in the computer BIOS.

Duck: I thought AGP still gets an IRQ, or maybe that's just PCI video cards...?
AGP does get an IRQ, which one depends on your motherboard/video card combination, eg. IRQ 10, IRQ 16. Although, AGP has it's own non-competing data bus.
Sometimes if I turn off "Allow IRQ for VGA" in BIOS, then the BIOS doesn't assign an IRQ, but don't remember if Windows does either, although it should.